Such a capacitive sensor assembly comprises at least one capacitive sensor element, which is formed by at least one (first) electrical conductor element as electrode and which is connected or is to be connected to an evaluation unit, which detects and evaluates at least one measured parameter depending on the capacitive coupling of the capacitive sensor element to its surrounding, as well at least one (potential leading) electrical reference conductor element, which forms or defines an electrical reference potential (zero potential) when determining said measured parameter or in more general during the capacitive coupling of the capacitive sensor element with its surrounding and which is arranged with a distance from the capacitive sensor element, thus in particular from the first electrical conductor element. The term conductor element is thereby at present being used in general for a singular or multipart electrical conductive element, which can be provided as a cable, flat conductor, an electrical conductor film or in any other form as electrical conductive component (electrical conducting assembly).
The assigned evaluation unit can determine and evaluate in particular the change of a measured value (for instance a measured current) depending on the capacitive coupling of a capacitive sensor element to a reference potential, which can be defined by a suitable vehicle element as for instance components of the vehicle body, components of a vehicle seat or the mentioned electrical heating element, wherein said capacitive coupling and thus said measured value is being influenced by the presence of a vehicle occupant or any other object in the surrounding of the capacitive sensor element. In case of a capacitive sensor element being integrated into a vehicle seat for instance the presence of a vehicle occupant being located on the seat can be herewith determined and also the occupant's condition can be concluded or a child seat can be recognized.
It is known from the U.S. Pat. No. 6,563,231 B1 to provide at least one electrical conductor element as electrode on a sensor mat for forming a capacitive sensor element. The sensor mat is being arranged as occupant detector on a vehicle part in order to be able to determine the occupation of a specific seat of a motor vehicle by a vehicle occupant. in concrete terms, the sensor mat with the electrical conductor element serving as capacitive sensor element is arranged for instance on a seat surface of the motor vehicle seat, and namely in particular between the seat cover and a seat cushion, that means somewhat below the seat cover and above the cushion foam. In order to determine, if a vehicle occupant is located on a vehicle seat being equipped with such a detector, the capacity between the sensor mat and a carrier of a reference potential (zero potential), for example defined by the vehicle floor (“earth”) or by at least one reference conductor element being integrated into the sensor mat, is being determined. This has by no means to occur imperatively directly by a (direct) capacity measurement; but it can occur by the measurement of any electrical values, which are linked to the mentioned capacity, the respective measured value thereof thus depends on the capacity (between the sensor mat and a carrier of the reference potential), as for instance by the measurement of measured currents (in or to the electrical conductor element serving as capacitive sensor element), compare also DE 100 48 956 C1 in case of a capacitive sensor assembly being integrated into a steering wheel.
If a vehicle occupant is located on the corresponding vehicle seat, then the resulting capacity is being influenced by the capacitive coupling of the occupant; and the resulting capacity corresponds to a combination (depending on the concrete situation for instance approximately a series connection) of the capacity between the seat surface (or the capacitive sensor element being provided there) and the vehicle occupant as well as the capacity between the vehicle occupant and the reference potential. By integrating thereby a reference conductor element with reference potential as counter electrode into the vehicle seat or into the sensor mat itself, the distance between the carrier of the reference potential and the vehicle occupant to be detected can be reduced and through this the measuring sensitivity can be improved.
With such an arrangement it can also be differentiated, if a vehicle occupant is located directly on the corresponding vehicle seat or if a children seat is attached to the vehicle seat, which in turn occupies a toddler. This determination can be used in order optionally to deactivate for instance by non-occupation or by occupation with a children seat one or multiple of the airbags assigned to the corresponding seat. Thus, a capacitive sensor assembly can be used as occupant detector in order to activate or deactivate safety devices of a motor vehicle, but also for triggering other functions of a motor vehicle, as for instance warning or control functions.
It is furthermore known from practice to arrange on a sensor mat beside at least one electrical conductor element serving as sensor element additionally electrically operating heating conductors, which are arranged with a distance from the first electrical conductor element and which, when under current, produce heat for heating a steering wheel, a vehicle seat or any other vehicle part.
In case of capacitive sensor systems, which comprise besides a capacitive sensor element also at least one electrical heating element and/or at least one reference conductor element, the compliance of a defined distance between the (first) electrical conductor element forming the capacitive sensor element as well as the heating conductors forming the electrical heating element or the respective reference conductor element is of importance in order to obtain reliable statements about the occupation of a seat with a vehicle occupant. In particular the effect of moisture and humidity on the measured values also depends on the distance between sensor element and heating element or sensor element and reference conductor element.